A conventional ventilating system of this type includes box-like ventilating fan body 103, motor 101 arranged in ventilating fan body 103, and blades 102 driven by motor 101. Ventilating fan body 103 is arranged on ceiling portion 104. The ventilating system of this type includes rotating speed detecting unit 105 that detects a rotating speed of motor 101 and current detecting unit 106 that detects a current flowing in the motor. Such a ventilating system controls motor 101 based on a rotating speed detected by rotating speed detecting unit 105 and a current detected by current detecting unit 106. Furthermore, a total ventilation air flow rate in ventilation performed in a period of a predetermined cycle is calculated based on the rotating speed detected by rotating speed detecting unit 105 and the current detected by current detecting unit 106. The calculated total ventilation air flow rate and a target total ventilation air flow rate of a predetermined cycle are compared with each other to calculate an excess or deficiency, and a ventilation air flow rate of a next cycle period is controlled such that a total ventilation air flow rate of the next cycle period is a value obtained by adding the excess or deficiency to the target ventilation air flow rate. This control is performed every cycle.
In the conventional ventilating system, as a drive motor that drives blades, a DC motor is frequently used, and an air flow rate is frequently set in a multi stage (for example, super high, high, and low). When the air flow rate of the ventilating system is set in a multi stage, for example, the air flow rate is consequently controlled in a very wide range, for example, air flow rate regions of 100 m3/h to 400 m3/h. In order to control an air flow rate in a very wide range and to control an air flow rate to set the air flow rate constant by using a current flowing in the motor and a rotating speed, the current needs to be detected more accurately.